In the field of optical communications, there are known numerous devices for performing switching of light by applying a voltage to a substance having an electro-optical effect to cause the refractive index thereof to change under a generated electric field. There have been proposed optical switches with waveguides constructed therein, including a directional-coupler-type optical switch which utilizes the proximity effect of two waveguides and a Mach-Zehnder-interferometer-type optical switch which utilizes an interference of light by applying a voltage between waveguides from an external source to generate a phase difference. These waveguide-type optical switches are capable of performing high-speed switching because they can change the refractive index at a high speed.
JP-A No. 2006-293018 proposes an optical switch comprising a crossed-Nicols optical system for rotating the plane of vibration of light which has been linearly polarized by birefringence due to a primary electro-optical effect (Pockels effect) or a secondary electro-optical effect (Kerr effect). Since this electro-optical optical switch employs an electro-optical effect, it is capable of operating as fast as the waveguide-type optical switches.
JP-A No. 2006-276654 proposes an optical switch which employs a nonlinear optical thin film including a fine crystal of a metal oxide that is excitable by visible light and which controls the reflection and transmission of incident light by inducing a reflecting phenomenon with visible exciting light applied from an external source.
There have been proposed numerous optical switches for changing incident light selectively to transmitted light and reflected light by applying a voltage to an electro-optical crystal to generate an electric field for inducing a change in the refractive index.
FIG. 1 shows the basic structure of a general optical switch which is relevant to the present invention. Two electrodes 1121 each made of an electron conductor are disposed in confronting relation to each other on respective side surfaces of electro-optical crystal 1104 in the form of a block. Two electrodes 1121 are connected to external power supply 1107 such that they have different polarities. When external power supply 1107 applies a voltage to two electrodes 1121, an electric field is generated between two electrodes 1121, producing refractive index changing portion 1108. When external power supply 1107 applies no voltage, the refractive index does not change between two electrodes 1121, and hence incident light 1101 travels straight through electro-optical crystal 1104 and is emitted out as transmitted light 1102. When external power supply 1107 applies a voltage, the refractive index changes between two electrodes 1121, and hence incident light 1101 which has an incident angle greater than the critical angle is reflected by electro-optical crystal 1104 and is emitted as reflected light 1103. By thus applying an appropriate voltage to electrodes 1121, the refractive index of electro-optical crystal 1104 is changed to switch between the transmission and reflection of light, thereby providing a function as an optical switch.
The directional-coupler-type optical switch and the Mach-Zehnder-interferometer-type optical switch described above need to be fabricated by a complex fabrication process because it is necessary to form waveguides in a crystal. Consequently, it is difficult to reduce the size of their devices and they are fabricated at a high cost. The optical switch comprising a crossed-Nicols optical system described above is problematic in that since the distance between the electrodes is large, an operating voltage is high and power consumption is large in order to obtain a desired rotational angle.
Patent document 1 discloses that the applied voltage is lowered by increasing the length of the optical path. However, as the increased length of the optical path requires a large crystal, the optical switch becomes highly costly, and the large crystal prevents the optical switch from being reduced in size and weight. The optical switch comprising a crossed-Nicols optical system does not lend itself to smaller device sizes because a phase difference is caused by the birefringence of the crystal, separately requiring a phase compensation wave plate. In addition, since the electrode length is increased and the electrode area is wider, the capacitance becomes greater and power consumption also becomes higher, making it difficult for the optical switch to operate at a high speed. Furthermore, the optical switch is disadvantageous in that it is not possible to obtain a high extinction ratio because the linearly polarized incident light is scattered by the crystal.
The optical switch disclosed in Patent document 2 cannot be reduced in size and weight because exciting light is separately required to excite the nonlinear optical film. Furthermore, the optical switch is disadvantageous in that since the signal light is switched by controlling the exciting light, the mechanism of the optical switch is more complex than a voltage control system employing an electro-optical crystal and is incapable of high-speed switching.
The optical switch according to the background art, in which a voltage is applied between the electrodes on the respective side surfaces of the electro-optical crystal to generate an electric field for inducing a change in the refractive index for changing incident light selectively to transmitted light and reflected light, suffers from structural problems in that it is difficult to reduce the thickness of the crystal to 100 μm or smaller due to limitations posed by crystal production or packaging, and the power consumption is large because a high drive voltage is required due to the large distance between the electrodes.
It is assumed that the power consumption is represented by P, the operating frequency by f, the capacitance between the electrodes on the crystal by C, the dielectric constant of vacuum by ∈0, the relative permittivity by ∈r, the beam diameter by r, the critical angle by θm, the length of the electrodes required to reflect a beam with the beam diameter r at the critical angle θm, the width of the electrodes by w, the interval between the electrodes by d, the refractive index by n0, the change in the refractive index by Δn, the Kerr constant by s, and the electric field E. These parameters are related to each other according to the following equations:
      P    =          2      ⁢      π      ⁢                          ⁢              fCV        2                  C    =                            ɛ          0                ⁢                  ɛ          r                ⁢        Lw            d            L    =          r              cos        ⁢                                  ⁢                  (                      θ            m                    )                                θ      m        =          a      ⁢                          ⁢      sin      ⁢              {                              (                                          Δ                ⁢                                                                  ⁢                n                            +                              n                0                                      )                                n            0                          }                        Δ      ⁢                          ⁢      n        =                  -                  1          2                    ⁢              n        0        3            ⁢              sE                  2          ⁢                                                          E    =          V      d      
From the above equations, the following equations (1), (2) are obtained:
                              θ          m                =                  a          ⁢                                          ⁢          sin          ⁢                      {                                                            α                  ⁡                                      (                                          V                      d                                        )                                                  2                            +              1                        }                                              (        1        )                                P        =                  β          ⁢                                          ⁢          d                                    (        2        )            where α, β represent coefficients, respectively.
It can be understood from the equation (1) that in order to obtain the same critical angle, the drive voltage V becomes higher in proportion to the distance d between the electrodes. It can also be understood from the equation (2) that power consumption P becomes higher in proportion to distance d between the electrodes.
General optical switches used for optical communications are required to have an extinction ratio of about 10:1, which is not sufficient in applications to image display devices such as displays. The optical switch shown in FIG. 1 which is relevant to the present invention has the same problems as the optical switches for use in optical communications in that it is very difficult for refractive index changing portion 1108 to fully reflect the incident light totally, and no sufficient extinction ratio can be achieved because the incident light is divided into a transmitted component and a reflected component.